no sound when I play video

I've looked in settings/my device/sound
Everything is turned up.
All my system sounds work, ringtones, alarm, etc
I first noticed it tonight when I played a vid here on ac
I have subsequently played several videos from YouTube and Facebook.
The video plays fine, but there is no sound.
I've swapped batteries, restarted numerous times.
Suggestions on where I might look to solve this?
Joe

What I did was to go to the Facebook app. Click on the menu button (the three bars topright corner). Scroll down to app settings. Scroll down to "delete cache for fb opened internet pages" and delete this cache. Then I selected and deselected the option "use 3rd party browser app". Then I closed the app. Killed all background running apps. Went to phone settings. App settings. Find facebook. And deleted the cache there aswell.
Now clicking a YouTube linked video in Facebook wall, opens in Facebook inhouse browser plays the embedded YouTube video with! audio again.
Nice!

I think the problem is most likely an operator malfunction.
I got caught on this myself.
I went thru the whole press the speaker / erase the cache partition / factory reset
bit and none of it worked.

Android is a new thing and the applications I have seen are not polished yet.
(Example: having to use the Gallery to select a contact photo.)

This assumes the Galaxy S8 but should be similar on other Android cellphones.
I am going to type this as if you want to play back a video you shot (with sound).

Tap 'Gallery' then 'Camera'.
You will see thumbnails of the videos you have taken.

Tap a video thumbnail.
The video will appear to be playing in a loop without audio.
There will be a 'play' icon in the middle of the video.
Tap the 'Play' icon in the middle of the screen before it disappears.
The video will start playing with audio.

On my Galaxy S4, only the sound in videos wasn't working. I opened the settings while the video was playing (touch the spot to the left of the home button at the bottom and a dialog box will appear) and there was an option to "listen via device" or something like that. The sound started playing immediately.